1. Field of the Invention
The present invention relates to a switched-capacitor input circuit. More particularly, the present invention relates to a switched-capacitor input circuit suitable for low supply voltage operation with respect to a single-ended input of a large signal, and to a switched-capacitor amplifier using the input circuit.
2. Description of the Related Art
As an input circuit of a switched-capacitor amplifier, which is configured to operate by receiving a single-ended input of a large signal, a double-sampling switched-capacitor input circuit illustrated in FIG. 6 has been known. The switched-capacitor input circuit has an object to facilitate input to a fully differential amplifier by converting the single-ended input signal into differential input signals having a constant common mode voltage.
A switched-capacitor input circuit 40 of FIG. 6 includes switches configured to alternately input signals Vin and Vr, and a set of capacitors Csa configured to sample the signals, and the input signal Vin is sampled with respect to an intended reference voltage Vcm using the reference voltage Vr as a reference. The switched-capacitor input circuit 40 performs double sampling in which, although the common mode voltage of the single-ended input of the large signal varies significantly depending on the input signal, a common mode voltage of differential signals Vop and Von, which are output after the sampling, is kept constant.
First, the inputs Vin and Vr are input to the upper capacitor Csa and the lower capacitor Csa, respectively, and the outputs Vop and Von are biased to a reference voltage Vcm. Next, the outputs Vop and Von are decoupled from the reference voltage Vcm, and Vr and Vin are input to the upper capacitor Csa and the lower capacitor Csa, respectively. As a result, twice a difference voltage between Vin and Vr is output from the output, and the output common-mode voltage becomes the reference voltage Vcm, and is kept constant.
FIG. 7 is a circuit diagram for illustrating a related-art switched-capacitor amplifier. The related-art switched-capacitor amplifier has the structure in which a fully differential amplifier 20, which includes feedback capacitors between inputs and outputs thereof, is connected to the switched-capacitor input circuit 40. The switched-capacitor amplifier has a gain of 2×Csa/Cfb, which is twice the gain of a general switched-capacitor amplifier that does not use the double-sampling method, and such a switched-capacitor amplifier is suitably used for the purpose of amplifying a micro signal.
However, when an operating supply voltage of an amplifier is small with a large signal input, there is rather a need to attenuate the signal. The attenuation is made possible by making the feedback capacitors of the amplifier larger than the input capacitors Csa. However, when the double sampling method is adopted, the signal is amplified to twice, and hence the feedback capacitors need to be made even larger. Therefore, there is a problem in that the circuit scale becomes larger. Moreover, making the feedback capacitors Cfb larger increases a load on the amplifier, and as a result, a problem also arises in that an operating current is increased.